1. Field of the Invention
The present invention relates to a focus detection apparatus for a camera or the like.
2. Related Art
In a conventional focus detection apparatus for a single-lens reflex camera, two images of light beams which pass through two different pupil regions of a photographing lens are received by a pair of sensor arrays and are photoelectrically converted therein. A relative positional displacement of the image signals as output from the pair of sensor arrays is obtained to detect an amount of out-of-focus (defocus amount) of the photographing lens. In this case, since only a brightness distribution of a specific area of the object space is detected by the pair of sensors, a defocus amount cannot be detected in an object having no brightness distribution in the specific area.
A plurality of sensor pairs and the corresponding focus detection optical systems are prepared to extract brightness distributions of the plurality of areas. Therefore, focus detection for many objects has been attempted, as disclosed in Japanese Patent Publication No. 59-28886, Japanese Patent Laid-Open No. 62-212611, and Japanese Patent Application No. 62-234895 filed by the assignee of the present application.
When an accumulation type sensor is used, accumulation time must be optimally controlled in accordance with an object brightness level in order to cause an output signal from the sensor to fall within the dynamic range of a processing system. In a system having the plurality of sensor pairs, when the accumulation operations of all the sensors are completed within the same accumulation time, different brightness levels are obtained since spatial areas of the object extracted by the respective sensor pairs are different. Therefore, the range of image signals thus obtained from the sensors may not fall within the dynamic range of the processing system.
For example, if a given portion corresponding to an object has a high brightness level and the accumulation time is shortened in accordance with a sensor pair corresponding to the given portion, object patterns are rarely obtained from other sensors corresponding to dark portions, resulting in inconvenience.
When a brightness distribution of the plurality of object areas is extracted to perform different focus detection operations in units of areas, one of a plurality of focus detection results must be selected and used as a final defocus amount. In order to eliminate this drawback, Japanese Patent Laid-Open No. 62-212611 discloses a technique for selecting a sensor output having the highest contrast level of the plurality of sensor outputs and performing focus detection by using the selected sensor output.
When an actual phase difference type focus detection optical system is taken into consideration, different base line lengths as the reference for precision are often employed between a plurality of optical systems. In this focus detection apparatus, the contrast levels of the sensor outputs cannot determine the precision of detection results of the focus detection mechanisms. Therefore, wrong selection may be made if the selection is based on only contrast factors.